The present invention relates to methods, systems and apparatus for allocating transmission resources and transmitting data in mobile telecommunication systems.
Embodiments of the present invention can for example allocate of transmission resources to machine type communication (MTC) devices in cellular telecommunications networks having orthogonal frequency division multiplex (OFDM) based radio access technology (such as WiMAX and LTE).
Certain classes of telecommunications device, such as MTC devices (e.g. semi-autonomous or autonomous wireless communication terminals), support “low capability” communication applications that are characterised, for instance, by the transmission of small amounts of data at relatively infrequent intervals.
In many scenarios, it is preferable to provide terminals dedicated to such “low capability” communication applications with a simple receiver unit (or transceiver unit) having capabilities more commensurate with the amount of data likely to be transmitted to (or from) the terminal. This more limited capability contrasts with the capabilities of the conventional mobile telecommunications terminals, such as smartphones, which share access to the same telecommunications networks.
To support MTC terminals, it has been proposed to introduce a “virtual carrier” operating within a bandwidth of one or more “host carriers”: the proposed virtual carrier concept preferably integrates within the transmission resources of conventional OFDM based radio access technologies and subdivides frequency spectrum in a similar manner to OFDM. Unlike data transmitted on a conventional OFDM type downlink carrier, data transmitted on the virtual carrier can be received and decoded without needing to process the full bandwidth of the downlink OFDM host carrier. Accordingly, data transmitted on the virtual carrier can be received and decoded using a reduced complexity receiver unit: with concomitant benefits such as increased simplicity, increased reliability, reduced form-factor and lower manufacturing cost.
The virtual carrier concept is described in a number of co-pending patent applications (including GB 1101970.0 [2], GB 1101981.7 [3], GB 1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB 1101982.5 [7], GB 1101980.9 [8] and GB 1101972.6 [9]), the contents of which are incorporated herein by reference.
In one version of the virtual carrier concept a plurality of subcarriers are disposed at predetermined offsets from a central frequency, similar to the subcarrier structure in conventional OFDM: the central frequency may be viewed as characterising the whole virtual carrier. The virtual carrier central frequency is typically selected to be the central frequency of the host carrier.
Direct conversion receiver architectures are attractive for manufacturers of mass market communication devices, such as MTC devices, because of their simplicity and lower cost. The communications networks have therefore adapted their standards to permit the adoption of such architectures. One adaptation has been to stipulate that the standard should not use a frequency band where direct conversion receivers are likely to encounter disruptive levels of interference arising from imperfections in the receiver itself (i.e. self-mixing)—the unused frequency band is commonly referred to as the DC subcarrier, where DC stands for “direct current”: non-use of a frequency band is typically achieved by not transmitting a carrier wave at the frequency band in downlink transmissions from a base station in the communications networks.
An efficient operation of a wireless telecommunications system for MTC devices is therefore desirable.